Studies of Acanthamoeba myosin IA and IB indicate that the ATP- binding site is within 26-kDa of the NH-2-terminus and the regulatory, phosphorylatable serine is between 38 and 58 kDa of the NH-2-terminus. An actin-binding site is about 62-64 kDa from the NH-2-terminus. A second actin-binding site, that is unrelated to ATPase activity but serves to allow myosin I to cross-link actin filaments, is in the 27-kDa COOH-terminal domain. An 80-kDa NH-2-terminal peptide has actin-activated Mg2+-ATPase activity but a 62-kDa NH-2-terminal peptide does not, although it is still able to bind to F-actin. This suggest the importance of all or part of the domain between 62 and 80 kDa of the NH-2-terminus. Cleavage between the ATP-binding site and the phosphorylation site (at about 38 kDa from the NH-2- terminus) produces a non-covalent complex in which the ATP and actin-binding sites can still affect each other but phosphorylation of the regulatory serine no longer affects either actin-binding or actin-activated Mg2+-ATPase activity. Additional evidence has been obtained that phosphorylation at the tip of the tail of myosin II regulates actin-activation Mg-2+- ATPase activity of the globular heads by altering the conformation of the filament as a whole. Both phosphorylated and dephosphorylated molecules were shown to be active as long as they are in filaments containing an appropriate level of dephosphorylated tails. Antibodies raised against synthetic peptides corresponding to specific sequences in the tip of the tail have identified a domain that is important for filament formation and consequent enzymatic activity. From electric birefringence studies, it has been possible to deduce the solution structure of myosin II monomers and dimers and propose a possible mechanism of filament formation that leads to a structure consistent with electron microscopic images. An Acanthamoeba kinesin has been purified with very high microtubule-activated Mg-2-ATPase activity and the ability to translocate latex beads at a high rate from the minus to the plus end of microtubules.